GCMS sensitivity question

[emma2009]

Hi everyone,

I am using a 5972A. We just installed this MS a few days ago. And we pumped it down, standard spectra tuned it, leak checked, it went pretty well. EM is at 1447 and no leaks showing up in the leak/air check. The foreline pressure is a bit higher than we wanted, it is about 75 mtorr, but the system still runs. Then I did a calibration, but it seems that the area counts are 5X lower than what we got from the last MS. The last MS was replaced by this one coz the EMV was at high level of 2800 and could not be decreased by all means. Does anyone know why I could only get this low sensitivity using this newer MS?

I am a newbee in this, so I wonder if I can increase the EM value manually so I can get a a higher sensitivity? I am not even sure if this is the right approach. Does anyone know? Is 75mtor a problem? We tried tighten everything, but that the best we could get and there were leaks, so we jsut went ahead running it.

Thanks a lot for your help...

Emma

[Steve Reimer]

Was the old MS also a 5972A? If not, the area counts are not necessarily going to be the same. It is better to look at the signal to noise, especially before turning up the EM voltage. Agilent has a discussion on this in the tech help section of their website. Increasing EM voltage increases noise as well as signal so use with caution.
If I had 2800 volts for my EM after running auto tune I would be ordering a new EM. 1500 volts is more reasonable.

[emma2009]

Thanks! The old one was also 5972A. I do understand the area counts may not be the same, but i wonder if there is a way to make it more sensitive coz I need a little bit more sensitive than this to serve my purpose.

Could you please send me the link to the discussion you mentioned? I find there is almost nothing about using ChemoStation software and diagnose problems we have... It is really appreciated if you have any information material about this software to share with us.

Thanks again!

Emma

[Steve Reimer]

http://www.chem.agilent.com/Library/Sup ... F05013.pdf

I ran into problems with sensitivity loss for brominated compounds in VOA analysis on my 5975. While looking for that I ran across the above discussion. It is directed at 5973 but the response issues will be similar for your older instrument.

It has been a while since I had to deal with a 5972, lots of things can cause a major loss of sensitivity including simple things like; is the column end located correctly? Is your foreline pressure as high as you think it is (diff pump heating properly)?

Good Luck!

[carras]

Hi, emma. The 5972 will run (meaning you can tune it, run mass profiles, etc) when the foreline pressure is lower than 120 mtorr or so. But I think that 75 mtorr is too high so you probably have a leak or the diffusion pump is not working properly (too cold, not enough oil in it, ...). As for leaks, I've had them in the big O-ring where the analyser seats.
Another thought. What helium flow rate are you using? The 5972 can cope with flows of 1.0 (perhaps 1.5) ml/min, if yours is higher than that you get high foreline pressure and lower sensitivity.

[thohry]

If your 5972 passes the autotune properly, I think the problem is somewhere between the transferline and GC. 75mtorr is a bit high, but not too hight that reduces the sensitivity 5 times.

[sllimbri]

Emma,
If your air water leak check is < 10 for all, then your ok. The 75mTorr is a reading from the silver foreline gauge. It has a small filament that will wear over time. If you do nothing but change that gauge, the foreline pressure will change. A reading of 50 and a reading of 100 will not affect the sensitivity. As a general rule of thumb, raise your multiplier by 100 and you will double your response. You must have a new multiplier, to autotune at 1447. Just bump it up and you'll be fine. Keep your autotune reports and you will be able to track the foreline gauge pressure over time. You'll see it increase.

If you need to replace the foreline gauge, look here.

http://www.helixtechnology.com/products/category50.html

[AICMM]

Emma, raise the EM voltage. People do it all the time. This is a perfectly legitimate way to improve sensitivity although you have to realize you also increase the noise when you raise the EMV.

On the other hand, lost sensitivity is often due to poor vaporization at the front and not to MS conditions. This is why keeping and comparing autotune reports (and respective standard chromatograms) is so important. Doing so gives you a good feel for MS conditions versus overall system response.

With all due respect here, I would strongly recommend that someone in your group takes an Agilent training class on MS or you bring someone in for training. I think it would be well worth the money spent.

Best regards,

AICMM